Is it pregnant? You can tell, if its pregnant, its VERY fat. How are the other fish? What about the ammonia, nitrite, nitrate? Does it have any physical symptoms like bite marks, spots, holes or anything?
Water changes. Lots of small water changes. Maybe one a day or every other day until the nitrate is reasonable (around 20ppm imo) and then keep up with your weekly 20% water changes from now on to prevent this happening again. This is assuming you don't have nitrate in your tap water, if you do then you'll have to try something else like a nitrate removing resin.
when fish are giving birth they dont sit on the bottom they will hide in plants for awhile and then they will swim around kind of strange trying to get the fry out. it sounds like your fish is sick. your molly sounds very small for 6 months old. is it still on the bottom? how much have you been feeding it? it could be constipated from too much food. or maybe something else is wrong.